Field
Embodiments of the disclosure relate generally to the field of oil, mining and water well logging systems and more particularly to a system incorporating an inertial measurement unit (IMU) platform incorporating a single axis gyroscope for measurement of rotation of the platform with an associated control system and motor to stabilize the platform thereby allowing reduction in the required dynamic range of the measurement gyroscopes in the IMU which in turn improves the measurement accuracy at low rates.
Background
Navigational measurement or “logging” of oil and water wells is of significant importance in modern drilling technology. Boreholes, particularly in directional drilling applications will vary significantly in angle and direction. Identifying the actual location and shape of a borehole may be critical and logging has been developed to accommodate that requirement. Typical logging systems employ inertial navigation devices incorporated in an instrument sonde which traverses the borehole. Two classes of inertial navigation system (INS) are well known. A platform inertial navigator uses mechanical gimbals and gyroscopes to space stabilize and maintain a set of accelerometers pointed in a constant frame of reference so that the direct double integration of the accelerometer measurements result in the position measurement of the body in motion relative to its initialize position. The attitude or Roll, Pitch and Yaw can be measured directly by the gimbal angles given the gimbal angles were aligned properly at the start of travel. A second class of INS is the strap down inertial navigator which eliminates the complexity of the mechanical gimbals using a set of gyroscopes to simply measure the Roll, Pitch and Yaw using the gyro derived angle measurements to mathematically translate the acceleration measurement into the frame of reference for double integration resulting in position. Some inertial systems designated Attitude Reference Systems (ARS) measure only attitude whiles the INS measures both the attitude and the position of the body in motion.
Each of these methods have certain advantages and disadvantages but typically the strap down system hardware is much simpler and lower cost while the computer programs and mathematics are more complex than a platform system. The gyroscopes used in a strap down system must be able to measure very large rates of motion when the body is moving and also very small rates of motion when the INS measures the Earth's spin rate in the process of finding its initial attitude. The ability to measure both small rates as well as high rates with the same device implies a very large dynamic range of measurement. Typically, the accuracy of the measurement made by a sensor is inversely proportional to the dynamic range of the device so a device that can measure very high rates has a difficult time measuring very low rates very precisely and accurately.
In the oil and mining industry, both platform and strap down systems have been developed and used commercially to survey and map the path of a borehole. Platform systems are not in wide spread application primarily due to their large diameter which limits their use to very large diameter boreholes. Strap down INS systems are more readily used in the industry but are still limited because of either size or accuracy limitations. The large gyros that are necessary for the highest accuracy systems make the systems too large for most applications. Systems employing small gyroscopes suffer from accuracy limitation due to the small size of the gyroscopes made worse by the large dynamic range required for strap down navigation mechanizations.
It is therefore desirable to provide a logging system which may employ small size gyroscopes in both ARS system as INS systems while improving accuracy of navigation.